Rubber composition for tires, and pneumatic tire

ABSTRACT

The present invention aims to provide a rubber composition for tires that can improve processability, crack resistance, ozone resistance, discoloration resistance, and tire appearance while maintaining a good balance between them, and also to provide a pneumatic tire formed from the rubber composition. The present invention relates to a rubber composition for tires, containing: a rubber component; an antioxidant; and polyether, the rubber component having a combined content of polybutadiene rubber, natural rubber, and polyisoprene rubber of 80% by mass or more per 100% by mass of the rubber component, the rubber composition containing the antioxidant in an amount of 0.3 to 7.0 parts by mass per 100 parts by mass of the rubber component, the polyether having an average mass content of at least two consecutive polyethylene oxide units of 85% or less of the polyethylene oxide mass content of the polyether.

TECHNICAL FIELD

The present invention relates to a rubber composition for tires and a pneumatic tire formed from the rubber composition.

BACKGROUND ART

Automobile tires, which are formed from rubber compositions containing natural rubber and synthetic diene rubbers as raw materials, can be degraded in the presence of ozone and eventually cracked. In order to reduce formation and growth of cracks in the presence of ozone, the rubber compositions contain additives, including, for example, antioxidants such as N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) or poly(2,2,4-trimethyl-1,2-) dihydroquinoline (TMDQ), and petroleum-derived waxes.

The antioxidants and petroleum-derived waxes in rubber vulcanizates migrate (bloom) to the rubber surface of e.g. tires, thereby protecting the rubber from ozone. However, excessive blooming of the antioxidants and petroleum-derived waxes in short periods of time causes white discoloration. Moreover, the antioxidants degraded by ozone cause brown discoloration and similarly can excessively bloom, resulting in a more intense brown discoloration. In addition, if the waxes and other agents bloomed to the surface of a tire form surface irregularities, diffused reflection of light will occur, with the result that the brown discoloration caused by the degraded antioxidants may be more noticeable, and moreover the tire will lose its shine.

Patent Literature 1 discloses the incorporation of a polyoxyethylene ether nonionic surfactant to prevent deterioration of tire appearance. Unfortunately, this surfactant can accelerate cure to cause problems such as scorch. Thus, there is still room for improvement in terms of the balance between processability and discoloration resistance.

CITATION LIST Patent Literature

Patent Literature 1: JP H05-194790 A

SUMMARY OF INVENTION Technical Problem

The present invention aims to solve the above problems and provide a rubber composition for tires that can improve processability, crack resistance, ozone resistance, discoloration resistance, and tire appearance while maintaining a good balance between them, and also provide a pneumatic tire formed from the rubber composition.

Solution to Problem

As a result of extensive studies to solve the problems, the inventors of the present invention succeeded in preparing a rubber composition for tires that has excellent discoloration resistance while reducing acceleration of cure, by reducing the average content of at least two consecutive polyethylene oxide units in a polyether while maintaining the amount of the polyether in the rubber compound. Thus, the inventors arrived at the present invention.

The present invention relates to a rubber composition for tires, containing: a rubber component; an antioxidant; and polyether (A), the rubber component having a combined content of polybutadiene rubber, natural rubber, and polyisoprene rubber of 80% by mass or more per 100% by mass of the rubber component, the rubber composition containing the antioxidant in an amount of 0.3 to 7.0 parts by mass per 100 parts by mass of the rubber component, the polyether (A) having a content of consecutive ethylene oxide units of 85% or less.

The term “content of consecutive ethylene oxide units” refers to the proportion of at least two consecutive polyethylene oxide units relative to the total polyethylene oxide content of the polyether, and is calculated using the following equation: Content of consecutive ethylene oxide units (%)=(Average number of at least two consecutive ethylene oxide units)/(Total number of ethylene oxide units)×100(%)

The term “at least two consecutive polyethylene oxide units” refers to at least two ethylene oxide (EO) units sequentially arranged as represented by, for example, -EO-EO- (two consecutive units) or -EO-EO-EO- (three consecutive units).

For example, in the case of a polyether including four ethylene oxide (EO) monomers and one propylene oxide (PO) monomer, the content of consecutive ethylene oxide units is calculated as follows.

TABLE 1 Number of at Probability least two Number of of indicated consecutive inconsecutive Arrangement arrangement E0 units E0 units HO-PO-E0 -E0 -E0 -E0 -H 1/5 4 0 HO-E0 -PO-E0 -E0 -E0 -H 1/5 3 1 HO-E0 -E0 -PO-E0 -E0 -H 1/5 4 0 HO-E0 -E0 -E0 -PO-E0 -H 1/5 3 1 HO-E0 -E0 -E0 -E0 -PO-H 1/5 4 0 Content of consecutive ethylene oxide units (%)=(Average number of at least two consecutive EO units)/(Total number of EO units)×100(%)=(⅕×4+⅕×3+⅕×4+⅕×3+⅕×4)/4×100(%)=3.6/4×100=90(%)

The rubber composition preferably contains the polyether (A) in an amount of 0.1 to 8.0 parts by mass per 100 parts by mass of the rubber component.

The polyether (A) preferably has a number average molecular weight of 1,500 to 50,000.

The rubber composition preferably contains 1 to 70 parts by mass of carbon black.

The present invention also relates to a pneumatic tire, including a tire component formed from the rubber composition.

The tire component is preferably at least one of a sidewall, clinch, or wing.

Advantageous Effects of Invention

The present invention provides a rubber composition for tires containing a rubber component, an antioxidant, and polyether (A), wherein the rubber component has a combined content of polybutadiene rubber, natural rubber, and polyisoprene rubber of 80% by mass or more per 100% by mass of the rubber component, the rubber composition contains the antioxidant in an amount of 0.3 to 7.0 parts by mass per 100 parts by mass of the rubber component, and polyether (A) has an average mass content of at least two consecutive polyethylene oxide units of 85% or less of the polyethylene oxide mass content of polyether (A). Such a rubber composition can improve processability, crack resistance, ozone resistance, discoloration resistance, and tire appearance while maintaining a good balance between them, and therefore can be used to produce a pneumatic tire excellent in the balance of these properties.

DESCRIPTION OF EMBODIMENTS

The rubber composition for tires of the present invention contains a rubber component, an antioxidant, and polyether (A). The rubber component has a combined content of polybutadiene rubber, natural rubber, and polyisoprene rubber of 80% by mass or more per 100% by mass of the rubber component. The rubber composition contains the antioxidant in an amount of 0.3 to 7.0 parts by mass per 100 parts by mass of the rubber component. Polyether (A) has an average mass content of at least two consecutive polyethylene oxide units of 85% or less of the polyethylene oxide mass content of polyether (A).

As described earlier, polyethers and other compounds containing ethylene oxide units may cause processability problems such as deterioration of scorch properties. This is probably because during e.g. vulcanization, the chelation of zinc ions by the consecutive ethylene oxide units promotes the reaction in which zinc ions are generated from zinc oxide, leading to acceleration of cure. In this regard, the present invention can reduce the acceleration of cure by reducing the content of consecutive ethylene oxide units.

When a specific polyether is incorporated into a rubber composition containing a specific rubber component and a specific amount of an antioxidant, the irregularities formed on the tire surface (bloom layer) by blooming of waxes and other agents can be flattened to reduce diffused reflection of light. This also results in improved discoloration resistance, such as decreases in the brown discoloration and white discoloration described above. Furthermore, excellent tire appearance is obtained, such as appropriate black appearance and shine imparted to the tire surface. At the same time, good crack resistance and good ozone resistance are achieved. Accordingly, processability, crack resistance, ozone resistance, discoloration resistance, and tire appearance can be improved while maintaining a good balance between them.

The rubber composition of the present invention contains at least one rubber selected from the group consisting of polybutadiene rubber (BR), natural rubber (NR), and polyisoprene rubber (IR). These rubbers may be used alone, or two or more of these may be used in combination. For a good balance between reinforcement and crack growth resistance, a combination of BR with NR and/or IR is more preferred.

The NR may be, for example, one usually used in the tire industry, such as SIR20, RSS#3, or TSR20. Any IR may be used, including those usually used in the tire industry. These rubbers may be used alone, or two or more of these may be used in combination.

The combined content of NR and IR per 100% by mass of the rubber component is preferably 30% by mass or more, more preferably 40% by mass or more. When it is less than 30% by mass, sufficient reinforcement or processability may not be ensured. The combined content of NR and IR is preferably 80% by mass or less, more preferably 70% by mass or less. When it is more than 80% by mass, sufficient crack growth resistance may not be ensured.

Any BR may be used, and examples include high-cis BR such as BR730 and BR51 both available from JSR Corporation, BR1220 available from Zeon Corporation, and BR130B, BR150B, and BR710 all available from Ube Industries, Ltd.; low-cis BR such as BR1250H available from Zeon Corporation; and BR containing 1,2-syndiotactic polybutadiene crystals (SPB) (SPB-containing BR) such as VCR412 and VCR617 both available from Ube Industries, Ltd. These rubbers may be used alone, or two or more of these may be used in combination.

In the case of the rubber composition containing BR, the content of BR per 100% by mass of the rubber component is preferably 10% by mass or more, more preferably 20% by mass or more. When it is less than 10% by mass, sufficient abrasion resistance or crack growth resistance may not be ensured. The content of BR is preferably 70% by mass or less, more preferably 65% by mass or less. When it is more than 70% by mass, reinforcement or handling stability may not be ensured.

SBR-containing BR can be used, for example, to impart reinforcement and fuel economy. In the case of the rubber composition containing SBR-containing BR, the 1,2-syndiotactic polybutadiene crystal content is preferably 5% by mass or more, more preferably 8% by mass or more. When it is less than 5% by mass, insufficient reinforcement and fuel economy may be imparted. The 1,2-syndiotactic polybutadiene crystal content is also preferably 22% by mass or less, more preferably 18% by mass or less. When it is more than 22% by mass, the SPB-containing BR may be too hard, thereby causing poor dispersion of rubber.

In the rubber composition of the present invention, the combined content of polybutadiene rubber, natural rubber, and polyisoprene rubber (preferably the combined content of polybutadiene rubber and natural rubber) per 100% by mass of the rubber component is 80% by mass or more, preferably 90% by mass or more, and may be 100% by mass. If it is less than 80% by mass, reinforcement, abrasion resistance, or crack growth resistance may not be ensured.

In addition to BR, NR, and IR, the rubber component may include any other rubber material. Examples include diene rubbers such as styrene-butadiene rubber (SBR), styrene-isoprene-butadiene rubber (SIBR), chloroprene rubber (CR), acrylonitrile butadiene rubber (NBR), ethylene-propylene-diene rubber (EPDM), butyl rubber (IIR), and halogenated butyl rubber (X-IIR). For example, SBR may be used to ensure reversion resistance and reinforcement. EPDM, butyl rubber, and/or halogenated butyl rubber may be used to impart weather resistance. These rubber materials may be used alone, or two or more of these may be used in combination.

The rubber composition of the present invention contains polyether (A) having a content of consecutive ethylene oxide units of 85% by mass or less.

The present invention can ameliorate the scorch problem by using polyether (A) having a content of consecutive ethylene oxide units of 85% by mass or less, probably for the following reason. In the case of, for example, a polyether consisting of ethylene oxide (EO) units and constituent units other than EO, such as propylene oxide (PO) units, the consecutive EO units having no side chain are more likely to form chelates with the zinc of zinc oxide than other units, thereby promoting cure so that scorch time decreases. By reducing the content of consecutive ethylene oxide units, the chelation can be reduced to improve scorch resistance.

The content of consecutive ethylene oxide units in polyether (A) is 85% by mass or less, preferably 80% by mass or less, more preferably 75% by mass or less. If the proportion of consecutive ethylene oxide units is more than 85% by mass of the total content of ethylene oxide units forming the polyether, the scorch-improving effect may decrease. The lower limit of the content of consecutive polyethylene oxide units is not particularly limited.

The polyether (A) preferably has a number average molecular weight (relative to styrene standards) of 300 or more, more preferably 1,500 or more, still more preferably 2,000 or more. Polyether (A) having a number average molecular weight of less than 300 may migrate to the rubber surface at too fast a rate (bloom rate), causing some problems, such as sticky rubber surface, excessive gloss, or poor appearance. The number average molecular weight (relative to styrene standards) of polyether (A) is preferably 50,000 or less, more preferably 40,000 or less, still more preferably 20,000 or less. Polyether (A) having a number average molecular weight of more than 50,000 may migrate to the rubber surface at too slow a rate, with the result that the irregularities on the tire surface are less likely to be flattened, and therefore the effect of improving tire appearance cannot be achieved.

Polyether (A) is represented by R¹—O—(R²—O)_(n)—R³ or R⁴—{(O—R²—O)_(n)—R¹}_(m), where R¹ and R³ each represent a hydrogen atom or a C1-C20 aliphatic hydrocarbon, alicyclic hydrocarbon, aromatic hydrocarbon, or alkanoyl group in which a hydrogen atom bound to a carbon atom may be replaced with an aromatic hydrocarbon group; each R² is the same as or different from each other and represents a C2-C10 divalent aliphatic hydrocarbon or divalent alicyclic hydrocarbon group in which a hydrogen atom bound to a carbon atom may be replaced with an aromatic hydrocarbon group; R⁴ represents a residue that is compound (a) having three or more hydroxyl groups from which the hydroxyl groups are removed; and n and m represent integers which are not limited as long as the content of consecutive polyethylene oxide units falls within the range indicated above.

Non-limiting examples of R¹ and R³ include methyl, ethyl, propyl, butyl, pentyl, hexyl, octyl, stearyl, oleyl, cyclohexyl, adamanthyl, phenyl, toluyl, naphthyl, pyridyl, acetyl, propanoyl, butanoyl, hexynoyl, octynoyl, octadecanoyl, oleoyl, and benzoyl groups.

Non-limiting examples of R² include ethylene, propane-1,2-diyl, butane-1,2-diyl, (2-methyl)propane-1,2-diyl, hexane-1,2-diyl, octane-1,2-diyl, (1-phenyl)ethane-1,2-diyl, (1-phenyl)propane-1,2-diyl, cyclohexane-1,2-diyl, and cyclooctane-1,2-diyl groups.

Non-limiting examples of compound (a) having three or more hydroxyl groups include glycerol, diglycerol, polyglycerol, monosaccharides, polysaccharides, trimethylolpropane, and pentaerythritol.

Specific examples of polyether (A) include polyalkylene glycols obtained by homopolymerization or copolymerization of oxirane compounds such as ethylene oxide, propylene oxide, 1,2-butylene oxide, 2,3-butylene oxide, 2-methylpropylene oxide, or cyclohexene oxide; compounds obtained by replacing the hydrogen atoms of the hydroxyl groups in the foregoing polyalkylene glycols with alkyl groups (e.g., polyoxyalkylene alkyl ethers, polyoxyalkylene dialkyl ethers); compounds obtained by dehydration reactions of the hydroxyl groups in the foregoing polyalkylene glycols with organic acids; and alkylene oxide adducts of polyhydric alcohols.

The amount of polyether (A) per 100 parts by mass of the rubber component is preferably 0.1 parts by mass or more, more preferably 0.3 parts by mass or more, still more preferably 0.6 parts by mass or more. When it is less than 0.1 parts by mass, too small an amount of polyether (A) will migrate to the rubber surface, and therefore the effect of improving tire appearance may not be achieved. The amount of polyether (A) is also preferably 8.0 parts by mass or less, more preferably 5.0 parts by mass or less, still more preferably 3.0 parts by mass or less. When it is more than 8.0 parts by mass, the effect of reducing acceleration of cure is less likely to be achieved, and in addition, the polyether may migrate to the rubber surface at too high a rate (bloom rate), causing problems such as poor appearance.

A specific amount of carbon black is preferably used in the present invention. This provides good reinforcement, resulting in good handling stability, crack resistance, and ozone resistance.

Specifically, the amount of carbon black per 100 parts by mass of the rubber component is preferably 1 part by mass or more, more preferably 15 parts by mass or more, still more preferably 25 parts by mass or more. An amount of less than 1 part by mass may not provide sufficient reinforcement. The amount of carbon black is preferably 70 parts by mass or less, more preferably 65 parts by mass or less. When it is more than 70 parts by mass, fuel economy tends to deteriorate, and in addition, the amount of polyether (A) adsorbed to carbon black will increase, which in turn reduces the amount of polyether (A) migrating to the rubber surface, with the result that the effect of improving tire appearance may not be achieved.

Non-limiting examples of carbon black include GPF, FEF, HAF, ISAF, and SAF. These materials may be used alone, or two or more of these may be used in combination.

The carbon black preferably has a nitrogen adsorption specific surface area (N₂SA) of 20 m²/g or more, more preferably 50 m²/g or more. Carbon black having a N₂SA of less than 20 m²/g may not provide sufficient reinforcement. The N₂SA is preferably 180 m²/g or less, more preferably 120 m²/g or less, still more preferably 90 m²/g or less, particularly preferably 80 m²/g or less. When it is more than 180 m²/g, processability may deteriorate, and in addition, the amount of polyether (A) adsorbed to carbon black will increase, and therefore the effect of improving tire appearance may not be achieved. The N₂SA of the carbon black is determined in conformity with JIS K 6217-2:2001.

The carbon black preferably has a dibutyl phthalate oil absorption (DBP) of 50 mL/100 g or more, more preferably 80 mL/100 g or more. Carbon black having a DBP of less than 50 mL/100 g may not provide sufficient reinforcement. The DBP of the carbon black is also preferably 200 mL/100 g or less, more preferably 135 mL/100 g or less, still more preferably 115 mL/100 g or less. When it is more than 200 mL/100 g, processability may deteriorate, and in addition, the amount of polyether (A) adsorbed to carbon black will increase, and therefore the effect of improving tire appearance may not be achieved. The DBP of the carbon black is determined in conformity with JIS K 6217-4:2001.

In the present invention, silica may be used together with a specific amount of carbon black. The incorporation of silica can further improve tire appearance. Non-limiting examples of silica include dry silica (anhydrous silicic acid) and wet silica (hydrous silicic acid). These materials may be used alone, or two or more of these may be used in combination. Preferred among these is wet silica because it has a larger effect in simultaneously achieving fracture resistance and wet grip performance.

The silica preferably has a nitrogen adsorption specific surface area (N₂SA) of 50 m²/g or more, more preferably 100 m²/g or more, still more preferably 150 m²/g or more. Silica having a N₂SA of less than 50 m²/g may not provide sufficient reinforcement. The N₂SA is preferably 250 m²/g or less, more preferably 210 m²/g or less. When it is more than 250 m²/g, processability may deteriorate, and in addition, the amount of polyether (A) adsorbed to silica will increase, and therefore the effect of improving tire appearance may not be achieved. The N₂SA of the silica is determined according to the BET method specified in ASTM D3037-93.

In the case of the rubber composition containing silica, the amount of silica per 100 parts by mass of the rubber component is preferably 3 parts by mass or more, more preferably 10 parts by mass or more, still more preferably 20 parts by mass or more. The amount of silica is also preferably 40 parts by mass or less. When the amount of silica is more than 40 parts by mass, the amount of polyether (A) adsorbed to silica will increase, and therefore the effect of improving tire appearance may not be achieved.

In the case of the rubber composition of the present invention containing silica, it preferably contains a silane coupling agent together with the silica.

Any silane coupling agent may be used which is conventionally used in combination with silica in the rubber industry. Examples include sulfide silane coupling agents such as bis(3-triethoxysilylpropyl)disulfide; mercapto silane coupling agents such as 3-mercaptopropyltrimethoxysilane; vinyl silane coupling agents such as vinyltriethoxysilane; amino silane coupling agents such as 3-aminopropyltriethoxysilane; glycidoxy silane coupling agents such as γ-glycidoxypropyltriethoxysilane; nitro silane coupling agents such as 3-nitropropyltrimethoxysilane; and chloro silane coupling agents such as 3-chloropropyltrimethoxysilane. These silane coupling agents may be used alone, or two or more of these may be used in combination. Among these, sulfide silane coupling agents are preferred, with bis(3-triethoxysilylpropyl)disulfide being more preferred.

In the case of the rubber composition containing a silane coupling agent, the amount of the silane coupling agent per 100 parts by mass of silica is preferably 2 parts by mass or more, more preferably 5 parts by mass or more. Less than 2 parts by mass of the silane coupling agent may not have a sufficient reinforcing effect. The amount of the silane coupling agent is also preferably 20 parts by mass or less, more preferably 12 parts by mass or less. An amount of more than 20 parts by mass tends not to have a reinforcing effect commensurate with the increase in cost.

A vulcanizing agent is used in the present invention in order to form crosslinks between polymer chains. Examples of the vulcanizing agent include those commonly used in the rubber industry, such as powdered sulfur, precipitated sulfur, colloidal sulfur, insoluble sulfur, highly dispersible sulfur, soluble sulfur, and alkylphenol-sulfur chloride condensates. These vulcanizing agents may be used alone, or two or more of these may be used in combination.

When the vulcanizing agent is sulfur, the amount of sulfur per 100 parts by mass of the rubber component is preferably 0.1 parts by mass or more, more preferably 0.5 parts by mass or more, still more preferably 1.0 part by mass or more. An amount of less than 0.1 parts by mass may not have a sufficient reinforcing effect. The amount of sulfur is preferably 6.0 parts by mass or less, more preferably 5.0 parts by mass or less, still more preferably 4.0 parts by mass or less, particularly preferably 3.0 parts by mass or less. An amount of more than 6.0 parts by mass may make the rubber excessively hard.

A wax is preferably incorporated in the present invention in order to reduce the formation and growth of cracks due to ozone. In the present invention, even when a wax is incorporated, the irregularities formed on the tire surface (bloom layer) by blooming of waxes and other agents can be flattened to reduce diffused reflection of light. This results in decreases in the brown discoloration and white discoloration described earlier. Furthermore, improved tire appearance is obtained, such as appropriate black appearance and shine imparted to the tire surface.

Non-limiting examples of the wax include petroleum-derived waxes and natural waxes. Synthetic waxes obtained by purification or chemical treatment of a plurality of waxes may also be used. These waxes may be used alone, or two or more of these may be used in combination.

Examples of petroleum-derived waxes include paraffin wax and microcrystalline wax. The natural wax may be any wax derived from non-petroleum resources, including, for example: vegetable waxes such as candelilla wax, carnauba wax, Japan wax, rice wax, and jojoba wax; animal waxes such as beeswax, lanolin, and spermaceti; mineral waxes such as ozokerite, ceresin, and petrolatum; and purified products of the foregoing waxes.

In the case of the rubber composition containing a wax, the amount of the wax per 100 parts by mass of the rubber component is preferably 0.5 parts by mass or more, more preferably 1.0 part by mass or more. An amount of less than 0.5 parts by mass may not provide sufficient ozone resistance. The amount of the wax is also preferably 12 parts by mass or less, more preferably 10 parts by mass or less, still more preferably 5.0 parts by mass or less. An amount exceeding 12 parts by mass may not be expected to have a larger effect in improving ozone resistance, but may increase the cost.

The rubber composition of the present invention may contain an oil. The incorporation of an oil improves processability and provides flexibility to tires, thereby allowing the effects of the present invention to be better achieved. Examples of the oil include process oils, vegetable oils and fats, and mixtures thereof. Examples of process oils include paraffinic process oils, aromatic process oils, and naphthenic process oils. Specific examples of paraffinic process oils include PW-32, PW-90, PW-150, and PS-32 all available from Idemitsu Kosan Co., Ltd. Specific examples of aromatic process oils include AC-12, AC-460, AH-16, AH-24, and AH-58 all available from Idemitsu Kosan Co., Ltd. Examples of vegetable oils and fats include castor oil, cottonseed oil, linseed oil, rapeseed oil, soybean oil, palm oil, coconut oil, peanut oil, rosin, pine oil, pine tar, tall oil, corn oil, rice oil, safflower oil, sesame oil, olive oil, sunflower oil, palm kernel oil, camellia oil, jojoba oil, macadamia nut oil, and tung oil. These oils may be used alone, or two or more of these may be used in combination.

In the case of the rubber composition containing an oil, the lower limit of the amount of the oil is not limited, and is preferably 60 parts by mass or less, more preferably 30 parts by mass or less, still more preferably 15 parts by mass or less, particularly preferably 10 parts by mass or less per 100 parts by mass of the rubber component. When the amount of the oil, which itself blooms to the tire surface, is within the range indicated above, blooming of polyether (A) can be suitably controlled, and therefore the effects of the present invention can be more suitably achieved.

The rubber composition of the present invention contains an antioxidant to reduce the formation and growth of cracks due to ozone. As described earlier, the present invention can reduce brown discoloration and white discoloration caused by the incorporation of antioxidants or other similar agents and thus can improve discoloration resistance and tire appearance.

Non-limiting examples of the antioxidant include naphthylamine antioxidants, quinoline antioxidants, diphenylamine antioxidants, p-phenylenediamine antioxidants, hydroquinone derivatives, phenol (monophenol, bisphenol, trisphenol, or polyphenol) antioxidants, thiobisphenol antioxidants, benzoimidazole antioxidants, thiourea antioxidants, phosphite antioxidants, and organic thioacid antioxidants. These antioxidants may be used alone, or two or more of these may be used in combination. Among these, p-phenylenediamine antioxidants are preferred because they provide good ozone resistance and allow the effects of the present invention to be more suitably achieved.

Examples of p-phenylenediamine antioxidants include N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine, N-isopropyl-N′-phenyl-p-phenylenediamine, N-1,4-dimethylpentyl-N′-phenyl-p-phenylenediamine, N,N′-diphenyl-p-phenylenediamine, N,N′-di-2-naphthyl-p-phenylenediamine, N-cyclohexyl-N′-phenyl-p-phenylenediamine, N,N′-bis(1-methylheptyl)-p-phenylenediamine, N,N′-bis(1,4-dimethylpentyl)-p-phenylenediamine, N,N′-bis(1-ethyl-3-methylpentyl)-p-phenylenediamine, N-4-methyl-2-pentyl-N′-phenyl-p-phenylenediamine, N,N′-diaryl-p-phenylenediamines, hindered diaryl-p-phenylenediamines, phenylhexyl-p-phenylenediamine, and phenyloctyl-p-phenylenediamine. These antioxidants may be used alone, or two or more of these may be used in combination. Among these, N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine is more preferred because it provides good ozone resistance and is also excellent in cost efficiency.

In the rubber composition, the amount of the antioxidant per 100 parts by mass of the rubber component is preferably 0.3 parts by mass or more, more preferably 0.5 parts by mass or more, still more preferably 1.0 part by mass or more. An amount of less than 0.3 parts by mass may not provide sufficient ozone resistance, and also is less likely to cause brown discoloration, with the result that the effects of the present invention are less likely to be achieved. The amount of the antioxidant is preferably 7.0 parts by mass or less, more preferably 6.0 parts by mass or less. When it is more than 7.0 parts by mass, a larger amount of the antioxidant may bloom and thereby deteriorate tire appearance, especially during initial use.

The rubber composition of the present invention preferably contains a vulcanization accelerator. Any type of vulcanization accelerator may be used including usually used ones. Examples of the vulcanization accelerator include sulfenamide vulcanization accelerators, thiazole vulcanization accelerators, thiuram vulcanization accelerators, thiourea vulcanization accelerators, guanidine vulcanization accelerators, dithiocarbamate vulcanization accelerators, aldehyde-amine or aldehyde-ammonia vulcanization accelerators, imidazoline vulcanization accelerators, and xanthate vulcanization accelerators. These vulcanization accelerators may be used alone, or two or more of these may be used in combination.

Examples of sulfenamide vulcanization accelerators include N-tert-butyl-2-benzothiazolylsulfenamide (TBBS), N-cyclohexyl-2-benzothiazolylsulfenamide (CBS), and N,N-dicyclohexyl-2-benzothiazolylsulfenamide (DCBS). These vulcanization accelerators may be used alone, or two or more of these may be used in combination.

In the case of the rubber composition containing a vulcanization accelerator, the amount of the vulcanization accelerator is not particularly limited and may be freely determined depending on the desired cure rate or crosslink density.

In order to accelerate cure, the rubber composition of the present invention may contain zinc oxide, stearic acid, or other similar agents. Any zinc oxide may be used, including those usually used in the tire industry such as zinc oxide #1 and zinc oxide #2 both available from Mitsui Mining and Smelting Co., Ltd.

In addition to the above-described components, the rubber composition of the present invention may appropriately contain other compounding agents usually used to prepare rubber compositions, such as stearic acid or tackifiers.

The rubber composition of the present invention can be prepared by any known method, such as by kneading the above components using a rubber kneading machine such as an open roll mill or Banbury mixer, followed by vulcanization.

The rubber composition of the present invention can be suitably used in tire components, especially, for example, sidewalls, clinches, and/or wings which form tire (outer) surfaces requiring good ozone resistance, discoloration resistance, and tire appearance.

A sidewall refers to a component located outside a carcass and extending from a shoulder portion to a bead portion. Specifically, it is a component as shown in, for example, FIG. 1 of JP 2005-280612 A and FIG. 1 of JP 2000-185529 A.

A clinch refers to a rubber portion located at a lower part of a sidewall and covering a portion to be in contact with a rim, and is also called a clinch apex or a rubber chafer. Specifically, it is a component as shown in, for example, FIG. 1 of JP 2008-75066 A.

A wing refers to a component located at a shoulder portion between a tread and a sidewall. Specifically, it is a component as shown in, for example, FIGS. 1 and 3 of JP 2007-176267 A.

The pneumatic tire of the present invention can be formed from the rubber composition by usual methods. Specifically, an unvulcanized rubber composition containing the above-described components is extruded and processed into the shape of a sidewall, a clinch, a wing, or other tire components, and then assembled with other tire components in a usual manner on a tire building machine to build an unvulcanized tire. The unvulcanized tire is heated and pressurized in a vulcanizer to produce a tire.

The pneumatic tire of the present invention can be used as, for example, a tire for passenger vehicles, a tire for trucks and buses, a tire for two-wheeled vehicles, or a high performance tire. The term “high performance tire” herein refers to a tire excellent particularly in grip performance, and conceptually includes racing tires for use in racing vehicles.

EXAMPLES

The present invention will be specifically described with reference to, but not limited to, examples.

The chemicals used in examples and comparative examples are listed below.

TABLE 2 NR RSS#3 IR IR2200 available from Zeon Corporation BR 1 BR150B (cis content: 97% by mass) available from Ube Industries, Ltd. BR 2 VCR412 (polybutadiene rubber containing 1,2-syndiotactic polybutadiene crystals (SPB-containing BR), 1,2-syndiotactic polybutadiene crystal content: 12% by mass) available from Ube Industries, Ltd, EPDM Esprene 505 (ethylene-propylene-diene rubber) available from Sumitomo Chemical Co., Ltd. IIR Butyl 065 (non-halogenated butyl rubber, isoprene content: 1.05 mol %) available from Exxon Mobil Corporation Carbon black Shoblack N330 (N₂SA: 75 m ² /g, DBP: 102 mL/100 g) available from Cabot Japan K.K. Silica Ultrasil VN₃ (N₂SA: 175 m²/g) available from Evonik Degussa Wax Sunnoc wax available from Ouchi Shinko Chemical Industrial Co., Ltd. Oil Process oil PW-32 (paraffinic process oil) available from Idemitsu Kosan Co., Ltd. Polyether 1 Polyether produced as described below Polyether 2 Newpol 50HB-100 (polyoxyethylene polyoxypropylene butyl ether, average number of moles of EO units added: 5, average number of moles of PO units added: 5) available from Sanyo Chemical Industries, Ltd. Polyether 3 Newpol 50HB-2000 (polyoxyethylene polyoxypropylene butyl ether, average number of moles of EO units added: 35, average number of moles of PO units added: 28) available from Sanyo Chemical Industries, Ltd. Polyether 4 Newpol GEP-2800 (polyoxyethylene polyoxypropylene glyceryl ether, average number of moles of EO units added: 24, average number of moles of PO units added: 24) available from Sanyo Chemical Industries, Ltd. Polyether 5 Newpol LB-1715 (polyoxypropylene butyl ether, number of PO units added: 40) available from Sanyo Chemical Industries, Ltd. Polyether 6 Newpol PE-64 (Pluronic nonionic surfactant (PEG/PPG-25/30 copolymer)) available from Sanyo Chemical Industries, Ltd. Polyether 7 PEG-4000N (polyethylene glycol) available from Sanyo Chemical Industries, Ltd. Polyether 8 Ionet T-80V (polyoxyethylene sorbitan monooleate) available from Sanyo Chemical Industries, Ltd. Polyether 9 Newpol 50HB-400 (polyoxyethylene polyoxypropylene butyl ether, average number of moles of EO units added: 12, average number of moles of PO units added: 12) available from Sanyo Chemical Industries, Ltd. Polyether 10 Newpol 50HB-660 (polyoxyethylene polyoxypropylene butyl ether, average number of moles of EO units added: 20, average number of moles of PO units added: 15) available from Sanyo Chemical Industries, Ltd. Antioxidant 1 Nocrac 6C (N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine) available from Ouchi Shinko Chemical Industrial Co., Ltd. Antioxidant 2 Nocrac 224 (2,2,4-trimethyl-1,2-dihydroquinoline polymer) available from Ouchi Shinko Chemical Industrial Co., Ltd. Stearic acid Stearic acid available from NOF Corporation Sulfur Powdered sulfur available from Tsurumi Chemical Industry Co., Ltd, Zinc oxide Zinc oxide #1 available from Mitsui Mining and Smelting Co., Ltd. Vulcanization Nocceler NS (N-tert-butyl-2-benzothiazolyl sulfenamide) available from accelerator Ouchi Shinko Chemical Industrial Co., Ltd. <Production of Polyether 1>

Into a pressure-resistant reaction vessel equipped with a thermometer, a heating/cooling system, a stirrer, and a dropping gas cylinder were introduced 90 parts of 1,2-butanediol (Wako Pure Chemical Industries, Ltd.) and six parts of potassium hydroxide (Tokyo Chemical Industry Co., Ltd.). After purging with nitrogen, the reaction vessel was sealed and heated to 140° C. The mixture was put under reduced pressure at 140° C. with stirring to remove two parts of water. Then, 2,910 parts of 1,2-butylene oxide (Tokyo Chemical Industry Co., Ltd.) was dropwise added over ten hours while adjusting the pressure to 0.5 MPa or lower, and the mixture was aged for five hours at the same temperature. After cooling to 60° C., six parts of acetic acid was introduced and stirred at 60° C. for 30 minutes to give 3,010 parts of polybutylene glycol. The polybutylene glycol had a hydroxyl value of 37.4 as measured according to the neutralization titration method specified in JIS K 0070, and also had a number average molecular weight of 3,000.

Table 3 shows the properties of Polyethers 1 to 10 used.

TABLE 3 Content of Ethylene consecutive Number oxide ethylene average content oxide units molecular ※1 ※2 weight Polyether 1 0.0 0.0 3000 Polyether 2 (Newpol 50HB-100) 43.2 66.7 510 Polyether 3 (Newpol 50HB-2000) 48.2 79.2 3200 Polyether 4 (Newpol GEP-2800) 37.8 73.4 2800 Polyether 5 (Newpol LB-1715) 0.0 0.0 2400 Polyether 6 (Newpol PE-64) 38.5 100.0 2850 Polyether 7 (PEG-4000N) 100 100.0 3100 Polyether 8 (Ionet T-80V) 84.3 100.0 1300 Polyether 9 (Newpol 50HB-400) 39.4 71.7 1340 Polyether 10 (Newpol 50HB-660) 48.9 79.9 1800 ※1 Ethylene oxide content of Polyether (Total mass of ethylene oxide units/Mass of Polyether) ※2 Content of consecutive ethylene oxide units (Average mass of consecutive ethylene oxide units/Total mass of ethylene oxide units)

According to each of the formulations shown in Tables 4 to 6, the chemicals other than the sulfur and vulcanization accelerator were kneaded using a 1.7 L Banbury mixer until the temperature of the mixture reached 180° C. Next, the sulfur and vulcanization accelerator were added to the kneaded mixture and they were kneaded using an open roll mill until the temperature of the mixture reached 105° C. to obtain an unvulcanized rubber composition. The unvulcanized rubber composition was then press-vulcanized at 160° C. for 15 minutes to obtain a vulcanized rubber composition.

The vulcanized rubber compositions prepared as above were evaluated as described below. Tables 4 to 6 show the results. Comparative Examples 1, 6, and 7 are taken as the reference comparative examples in Tables 4, 5, and 6, respectively.

<Evaluation of Scorch Resistance>

Scorch time (t10) at 100° C. was measured in conformity with JIS K 6300 “Evaluation procedure of unvulcanized rubber” to calculate an index ((scorch time (t10) of each formulation example)/(scorch time (t10) of reference comparative example)×100), with the reference comparative example set equal to 100. A higher index indicates better scorch resistance.

<Evaluation of Ozone Resistance>

A test piece with a predetermined size was prepared from each of the prepared vulcanized rubber compositions and subjected to a dynamic ozone deterioration test in conformity with JIS K 6259 “Rubber, vulcanized or thermoplastics—Determination of ozone resistance”. The test was carried out for 48 hours at a frequency of reciprocation of 0.5±0.025 Hz, an ozone concentration of 50±5 pphm, a test temperature of 40° C., and a tensile strain of 20±2%. After the test, crack conditions (the presence or absence of cracks) were analyzed to evaluate ozone resistance. A higher index indicates a smaller number of cracks and a smaller size of crack, and therefore better ozone resistance.

<Evaluation of Discoloration Resistance>

The samples after the ozone resistance test were analyzed with a chromameter (CR-310) available from Konica Minolta, Inc. to determine “a” and “b” values (L*a*b* color system). The value of (a2+b2)−0.5 was used as a parameter and expressed as an index calculated from the formula: (parameter of each formulation example)/(parameter of reference comparative example)×100, with the reference comparative example set equal to 100. A higher index indicates less discoloration and therefore better discoloration resistance.

<Evaluation of Appearance>

The appearance of the samples after the ozone resistance test was evaluated in an open air based on the following criteria.

AA: Darker black than reference comparative example and shiny

A: Darker black than reference comparative example and slightly shiny

B: Similar shade of brown as reference comparative example

C: Darker brown than reference comparative example

TABLE 4 Example Comparative Example 1 2 3 4 5 6 7 8 9 1 2 3 4 Formulation NR (RSS#3) 43 43 43 43 43 43 43 43 43 43 43 43 43 (parts by BR 1 (BR150B) 57 57 57 57 57 57 57 57 57 57 57 57 57 mass) Carbon black (N330) 60 60 60 60 60 60 60 60 60 60 60 60 60 Wax (Sunnoc wax) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Oil (Process oil PW-32) 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 Polyether 1 1.5 — — — — — — — — — — — — Polyether 2 (Newpol 50HB-100) — 1.5 — — — 3.0 — — — — — — — Polyether 3 (Newpol 50HB-2000) — — 1.5 — — — — — — — — — — Polyether 4 (Newpol GEP-2800) — — — 1.5 — — — — — — — — — Polyether 5 (Newpol LB-1715) — — — — 1.5 — 6.0 — — — — — — Polyether 6 (Newpol PE-64) — — — — — — — — — 1.5 — — — Polyether 7 (PEG-4000N) — — — — — — — — — — 1.5 — 6.0 Polyether 8 (Ionet T-80V) — — — — — — — — — — — 1.5 — Polyether 9 (Newpol 50HB-400) — — — — — — — 1.5 — — — — — Polyether 10 (Newpol 50HB-660) — — — — — — — — 1.5 — — — — Antioxidant 1 (Nocrac 6C) 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 Stearic acid 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 Sulfur (powdered sulfur) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Zinc oxide (zinc oxide #1) 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 Vulcanization accelerator 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 (Nocceler NS) Evaluation Scorch resistance 141 135 116 108 140 106 139 117 112 100 65 89 33 Ozone resistance 103 103 102 103 103 106 105 104 103 100 99 101 104 Discoloration resistance 100 102 103 104 102 105 101 104 102 100 99 97 94 Appearance B B A A B A B A A B B C C

TABLE 5 Example Comparative Example 10 11 5 6 Formulation NR (RSS#3) 50 50 50 50 (parts by BR 1 (BR150B) 27 27 27 27 mass) BR 2 (VCR412) 23 23 23 23 Carbon black (N330) 38 38 38 38 Wax (Sunnoc wax) 1.5 1.5 1.5 1.5 Oil (Process oil PW-32) 6.0 6.0 6.0 6.0 Polyether 3 (Newpol 50HB-2000) 1.5 — — — Polyether 5 (Newpol LB-1715) — 1.5 — — Polyether 6 (Newpol PE-64) — — 1.5 — Polyether 8 (Ionet T-80V) — — — 1.5 Antioxidant 1 (Nocrac 6C) 2.8 2.8 2.8 2.8 Antioxidant 2 (Nocrac 224) 0.5 0.5 0.5 0.5 Stearic acid 2.0 2.0 2.0 2.0 Sulfur (powdered sulfur) 2.1 2.1 2.1 2.1 Zinc oxide (zinc oxide #1) 3.7 3.7 3.7 3.7 Vulcanization accelerator 0.8 0.8 0.8 0.8 (Nocceler NS) Evaluation Scorch resistance 168 201 72 100 Ozone resistance 102 104 100 100 Discoloration resistance 104 102 101 100 Appearance A B B B

TABLE 6 Comparative Example Example 12 13 7 Formulation NR (RSS#3) 30 30 30 (parts by IR (IR2200) 10 10 10 mass) BR 1 (BR150B) 45 45 45 EPDM (Esprene 505) 15 15 15 Carbon black (N330) 62 62 62 Wax (Sunnoc wax) 1.5 1.5 1.5 Oil (Process oil PW-32) 6.0 6.0 6.0 Polyether 3 (Newpol 50HB- 0.2 — — 2000) Polyether 5 (Newpol LB- — 0.2 — 1715) Polyether 7 (PEG-4000N) — — 0.2 Antioxidant 1 (Nocrac 6C) 0.7 0.7 0.7 Stearic acid 2.0 2.0 2.0 Sulfur (powdered sulfur) 1.5 1.5 1.5 Zinc oxide (zinc oxide #1) 2.2 2.2 2.2 Vulcanization accelerator 1.0 1.0 1.0 (Nocceler NS) Evaluation Scorch resistance 104 106 100 Ozone resistance 100 100 100 Discoloration resistance 101 101 100 Appearance B B B

TABLE 7 Comparative Example Example 14 15 8 Formulation NR (RSS#3) 50 50 50 (parts by BR 1 (BR150B) 27 27 27 mass) BR 2 (VCR412) 23 23 23 Carbon black (N330) 30 30 30 Silica (Ultrasil VN₃) 10 10 10 Wax (Sunnoc wax) 1.5 1.5 1.5 Oil (Process oil PW-32) 6.0 6.0 6.0 Polyether 3 (Newpol 50HB- 1.5 — — 2000) Polyether 5 (Newpol LB- — 1.5 — 1715) Polyether 6 (Newpol PE-64) — — 1.5 Antioxidant 1 (Nocrac 6G) 2.8 2.8 2.8 Antioxidant 2 (Nocrac 224) 0.5 0.5 0.5 Stearic acid 2.0 2.0 2.0 Sulfur (powdered sulfur) 2.1 2.1 2.1 Zinc oxide (zinc oxide #1) 3.7 3.7 3.7 Vulcanization accelerator 0.8 0.8 0.8 (Nocceler NS) Evaluation Scorch resistance 235 275 100 Ozone resistance 105 104 100 Discoloration resistance 102 101 100 Appearance A A B

Processability (scorch resistance), crack resistance, ozone resistance, discoloration resistance, and tire appearance were improved while maintaining a good balance between them in the examples in which the rubber composition contained: a rubber component having a combined content of polybutadiene rubber, natural rubber, and polyisoprene rubber of 80% by mass or more; a predetermined amount of an antioxidant; and polyether (A) having a content of consecutive ethylene oxide units of 85% by mass or less. 

The invention claimed is:
 1. A rubber composition for tires, comprising: a rubber component; an antioxidant; and a polyether, wherein the rubber component has a combined content of polybutadiene rubber, natural rubber, and polyisoprene rubber of 80% by mass or more per 100% by mass of the rubber component, the antioxidant is present in an amount of 0.3 to 7.0 parts by mass per 100 parts by mass of the rubber component, the polyether has an average mass content of at least two consecutive polyethylene oxide units of 85% or less of the polyether polyethylene oxide mass content, the polyether is represented by R¹—O—(R²—O)_(n)—R³ or R⁴—{(O—R²—O)_(n)—R¹}_(m), wherein R¹ and R³ each represent a hydrogen atom or a C1-C20 aliphatic hydrocarbon, alicyclic hydrocarbon, aromatic hydrocarbon, or alkanoyl group in which a hydrogen atom bound to a carbon atom may be replaced with an aromatic hydrocarbon group; each R² is the same as or different from each other and represents a C2-C10 divalent aliphatic hydrocarbon or divalent alicyclic hydrocarbon group in which a hydrogen atom bound to a carbon atom may be replaced with an aromatic hydrocarbon group; R⁴ represents a residue that is compound (a) having three or more hydroxyl groups from which the hydroxyl groups are removed; and n and m represent integers which are not limited as long as the content of consecutive polyethylene oxide units falls within the range indicated above.
 2. The rubber composition for tires according to claim 1, wherein the rubber composition comprises the polyether in an amount of 0.1 to 8.0 parts by mass per 100 parts by mass of the rubber component.
 3. The rubber composition for tires according to claim 2, wherein the polyether has a number average molecular weight of 1,500 to 50,000.
 4. The rubber composition for tires according to claim 2, wherein the rubber composition comprises 1 to 70 parts by mass of carbon black.
 5. The rubber composition for tires according to claim 1, wherein the polyether has a number average molecular weight of 1,500 to 50,000.
 6. The rubber composition for tires according to claim 5, wherein the rubber composition comprises 1 to 70 parts by mass of carbon black.
 7. The rubber composition for tires according to claim 1, wherein the rubber composition comprises 1 to 70 parts by mass of carbon black.
 8. A pneumatic tire, comprising a tire component formed from the rubber composition according to claim
 1. 9. The pneumatic tire according to claim 8, wherein the tire component is at least one of a sidewall, clinch, or wing.
 10. A pneumatic tire, comprising a tire component formed from the rubber composition according to claim
 2. 11. A pneumatic tire, comprising a tire component formed from the rubber composition according to claim
 5. 12. A pneumatic tire, comprising a tire component formed from the rubber composition according to claim
 7. 